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Fiber optic sensor systemFiber optic sensor system description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060139652, Fiber optic sensor system. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD AND BACKGROUND OF THE INVENTION [0001] The present invention is generally related to seismic sensors, and more particularly to fiber optic seismic sensor systems. As used throughout this application and its appended claims, seismic movement and activity can mean any vibrations capable of measurement by a land-based sensor, whether caused by geologic activity, other natural phenomena, explosions, the motion or effects of mechanical implements or any other activity causing vibrations in a land mass. [0002] The traditional method for detecting seismic signals has been the coil-type geophone. Geophone sensors comprise a mass-spring assembly contained in a cartridge about 3 cm long and weighing about 75 grams. In a typical geophone sensor, the spring is soft and as the cartridge case moves the mass (coil) is held in place by its own inertia. Thus, the coil acts as a reference for measurement of the cartridge displacement. This sensor arrangement is used for measurement of large, oscillatory displacements on the order of millimeters with sub-micrometer resolution. The frequency range of these sensors is limited, however. For best sensitivity to small displacements, a given sensor has a mechanical bandwidth of about 10 Hz. Sensors can be designed with center frequencies from 20 Hz to 100 Hz. [0003] Micro-Electro-Mechanical Systems (MEMS) are miniature mechanical components fabricated in silicon wafers. The fabrication methods are based on the same photolithographic and etching processes used to manufacture electronic circuits in silicon. In fact, most MEMS devices include not only miniature mechanical components such as beams, nozzles, gears, etc. but also, integrated electronic components to provide local signal conditioning. Unfortunately, the integrated circuits limit the maximum operating temperature of electronic MEMS to 75.degree. C. The maximum temperature limit can be extended to 400.degree. C. or more if optical fiber sensors are integrated with mechanical MEMS components so that no electronics are needed in the high temperature environment. [0004] Recently, MEMS accelerometers have been developed for 3-component (3C) land seismic measurements. In the MEMS accelerometer, a mass-spring assembly is also used, but unlike the geophone, the spring is stiff and the mass moves with the case that houses the MEMS. The inertia of the mass causes strain and deflection of the spring and the deflection or strain can be measured with a sensor to determine the acceleration. High performance 3C MEMS accelerometers with capacitance sensors have been demonstrated. [0005] The measurement range of accelerometers is specified in units of `G` where 1G=9.8 m/s.sup.2. Commercial specifications include 120 dBV dynamic range (1G to 10.sup.-6 G) and 500 Hz mechanical bandwidth with 24-bit digital resolution equivalent to a noise limited performance of 10.sup.-7G/(Hz).sup.1/2. The accelerometer is fabricated on a silicon chip on the order of 100 mm.sup.2 and weighing roughly 1 gram. Three single-axis accelerometers (each with an application specific integrated circuit (ASIC) on each chip for signal conditioning) are packaged to measure in three orthogonal directions. The limitation of these accelerometers is an upper limit on the operating temperature of 75.degree. C., which is imposed by the electronic integrated circuits and is not a fundamental limitation of silicon itself. [0006] Additional objects and advantages are set forth in the description which follows, as well as other that may be obvious from the description, known to those skilled in the art or may be learned by practice of the invention. DESCRIPTION OF THE DRAWINGS [0007] Exemplary objects and advantages, taken together with the operation of at least one embodiment, may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein: [0008] FIG. 1a is a top-view diagrammatical representation of a MEMS cantilever which can be integrated within a silicon wafer, a frame or a combination thereof; [0009] FIG. 1b is a partial side-view representation of the interface between the MEMS cantilever and the optical fiber; [0010] FIG. 2 is a schematic representation of a basic optoelectronic signal processor; [0011] FIG. 3 is a graphical representation of the signal level versus gap when the source wavelength is near 1500 nm, source bandwidth is 4 nm, and the sensor gap is 150 .mu.m; and [0012] FIG. 4 is a diagrammatical representation of the system architecture for three MEMS accelerometer units, although this representation may be easily modified to accommodate additional MEMS accelerometer units. DETAILED DESCRIPTION [0013] While the present invention is described with reference to the preferred embodiment, it should be clear that the present invention should not be limited to this embodiment. Therefore, the description of the preferred embodiment herein is illustrative of the present invention and should not limit the scope of the invention as claimed. [0014] Reference will now be made in detail to a preferred embodiment illustrated in the accompanying drawings, which illustrate an accelerometer/sensor design and overall system architecture. [0015] The design of the sensor is based on integration of a interferometric fiber optic sensor with a MEMS accelerometer structure. Table 1 below summarizes typical design specifications for an interferometer integrated with MEMS accelerometer. Notably, a Fabry-Perot, two beam or other multiple beam type interferometer can be used in accordance with the invention described herein. TABLE-US-00001 TABLE 1 Operating Temperature 250.degree. C. Resolution 100 nano-G/(Hz).sup.1/2 at 100 Hz Bandwidth (mechanical) 5 Hz to 500 Hz Dynamic range 120 dB(V) (60 dB(G)) Other Measure in three orthogonal axes [0016] To accommodate the need for an accelerometer with a maximum sensitivity that has a nominal resonant frequency of 100 Hz, two MEMS units can be used on each axis in order to cover a wider G range as well as the mechanical bandwidth requirements, although both the number of units in each axis as well as the number of axes can be altered to suit the desired sensitivity of sensor. Assuming the displacement range for each accelerometer is 1 nm to 1000 nm, the performance characteristics for each MEMS are given in Table 2. TABLE-US-00002 TABLE 2 MEMS design parameters. G (9.8 m/s2) Displacement Max Resonant Frequency MEMS Max/Min (nm) Frequency Range (Hz) A 1/10.sup.-4 1,000 140 5-500 B 10.sup.-3/10.sup.-6 1,000 50 5-250 [0017] A diagram of the MEMS cantilever is shown in FIGS. 1a and 1b, while representative dimensions are provided below in Table 3. With this particular arrangement, the frequency response falls off below 10.sup.-4G, although further improvements to the response can be engineered. [0018] With reference to FIG. 1a, which is a top view of cantilever 10, and FIG. 1b, which is a side view of cantilever 10, optical fiber 12 delivers light to an end of the cantilever 10. The end of the fiber has a partially reflective coating labeled R, as does the silicon cantilever, so as to define an interferometer cavity (as mentioned above) where the reflectors are spaced apart by gap distance labeled g. As g changes, changes in the relative phase of the light reflected from the end of the fiber and the cantilever produce changes in light interference that modulate the total light signal reflected back into the fiber. [0019] Understanding that the cantilever 10 will be mechanically coupled to the MEMS unit which houses the interferometer, the position of the fiber 12 relative to the cantilever 10 (that itself defines the gap distance g) should be selected so as to maximize sensitivity to vibrations of the cantilever 10. For the ease of construction and stability during operation, it may be preferable to etch the cantilever onto a planar wafer such that the "leg" portions (shown in FIG. 1a as having a width `w`) are attached to a larger assembly or frame, thereby imparting a U-shape to the cantilever. Other shapes providing one or more anchor points may also be possible, including but not limited to an E-shape, a Y-shape and the like. Also, if the cantilever is constructed from silicon or some other at least partially reflective material, the cantilever does not necessarily need to have reflective coating deposited thereon (although use of a non-reflective coating, a reflective coating, a partially transmissive coating or any combination thereof, whether on the interior or exterior of the cantilever, may be used to add further precision). Clearly, ease of manufacture for the cantilever can be achieved by constructing the cantilever from silicon, silicon carbide or other materials commonly used in MEMS devices. [0020] Table 3 lists the MEMS dimensions when the cantilevers have a preferred uniform thickness of 25 .mu.m. Note that the information below corresponds to the reference lines indicated in FIG. 1a. TABLE-US-00003 TABLE 3 MEMS a (mm) b (mm) c (mm) w (mm) A 4 4 3 0.1 B 8 4 2 0.1 [0021] A preferred optoelectronic signal processor schematic for the invention is shown in FIG. 2. This processor is designed to provide input to software so as to monitor the signals from the MEMS units. Continue reading about Fiber optic sensor system... Full patent description for Fiber optic sensor system Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Fiber optic sensor system patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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